In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a variety of U.S. patents assigned to the assignee of this patent application, including U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,641,135; 5,642,122; and 5,650,788. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth orbit (LEO) satellites that implement an Earth-fixed cellular beam approach to transmitting data from one location on the Earth's surface to another location. More specifically, each LEO satellite has a communication "footprint" that covers a portion of the Earth's surface as a satellite passes over the Earth. The communication footprint defines the area on the Earth within which ground terminals can communicate with the satellite. Located within each footprint are a large number of cells. During the period of time a cell remains within the borders of a satellite footprint, ground terminals located in the cell transmit data to and receive data from the serving satellite. When a satellite reaches the end of its serving arc, another orbiting satellite is positioned to serve the Earth-fixed cell previously covered by the satellite reaching the end of its serving arc. During serving, the antennas of ground terminals located within the cells continuously point toward the serving satellite as it moves in orbit and antennas on the satellite point toward the cell during the time period within which the cell is allowed to transmit data.
Data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within the cell to the satellite serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites to the satellite serving the cell within which the ground terminal of the designated receiver is located. The latter satellite transmits the data to the receiver ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a satellite data communication network wherein each ground terminal and satellite forms a node of the network.
In order for a LEO satellite data communication system to be competitive, it must have a wide bandwidth. In the United States, the frequency spectrum is crowded at lower frequencies due to pre-allocated terrestrial and satellite users. Such a wide bandwidth is therefore generally only available in the gigahertz (GHz) range. In order to be competitive, it is also advantageous that the satellite data communication system be designed such that each ground terminal is able to rapidly acquire and synchronize its operation to the satellite serving the ground terminal. The orbit of the satellite in a LEO constellation causes each satellite to be visible to a ground terminal for only a limited period of time. In order to maximize the amount of time (and hence data) that the ground terminal can communicate with each satellite, it is therefore important to minimize the acquisition and synchronization period. The present invention is directed to a satellite acquisition and synchronization system that accomplishes these objectives.